1. Field of the Invention
The invention relates to a process and apparatus for the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor. The benefits of riser reactor FCC units are such that many older units have been revamped to take advantage of this advance in technology.
There have been many improvements in the design of FCC regenerators. The considerable evolution in the design of FCC units is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article.
Most new regenerators are of the high efficiency design, i.e., the spent catalyst, preferably with recycled regenerated catalyst, is charged to a fast fluidized bed coke combustor, and from their to a dilute phase transport riser. Coke is efficiently burned in the robustly fluidized coke combustor, while CO afterburning is promoted by the dilute phase conditions in the transport riser. Such regenerators are now the standard for new construction, and are shown in U.S. Pat. No. 4,820,404, Owen, U.S. Pat. No. 4,353,812, Lomas et al, and many others. These two patents are incorporated herein by reference.
These are excellent regenerators, which greatly reduce the amount of catalyst inventory needed to process a given amount of fresh feed. The only areas where such regenerators sometimes have problems is particulates emissions, and somewhat higher pressure drop than is desirable.
Dust emissions can be a problem in some areas with these regenerators. They are somewhat "dusty" because all of the catalyst inventory is discharged as a dilute phase up the transport riser to an outlet in a dilute phase region above a second fluidized bed. High vapor velocities are needed to get the catalyst, the volume of which is usually multiplied by 100 to 300% due to catalyst recycle, up through the coke combustor and transport riser. Adding cyclones to the transport riser outlet can greatly reduce particulates emissions due to such large amounts of catalyst through the transport riser, but cyclones add to the cost and complexity of such units, and cyclones also add to the pressure drop across the regenerator.
Even when cyclones are not needed, there is a significant pressure drop, and a significant amount of work, involved in moving extremely large volumes of catalyst around such units. Although the benefits or reduced catalyst inventory, and more efficient regeneration of catalyst, and dilute phase controlled afterburning of CO to CO2 in the transport riser, when desire, are worth the pressure drop, it would be beneficial if the desirable regeneration characteristics of such units could be retained, but without all the pressure drop (or energy consumption) required to get catalyst up through a fast fluidized bed coke combustor and a dilute phase transport riser.
Such modern regenerator designs, sometimes called a high efficiency regenerator, are preferred for all new construction. For the many FCC units built with low efficiency, i.e., bubbling dense bed regenerators, it has not been possible and/or economically justifiable to improve the efficiency of the bubbling bed regenerator.
Such bubbling bed regenerators are inherently inefficient because of the presence of large gas bubbles, poor catalyst circulation, and the stagnant regions. The bubbling bed regenerators usually have two to three times the catalyst inventory of more modern regenerators. The increased inventory, and longer catalyst residence time, make up for a lack of efficiency.
For such units, characterized by a single, bubbling dense bed regenerator, there has been no good way to achieve the benefits of high efficiency regeneration. Site constraints and capital spending constraints usually prevent replacement of a bubbling bed regenerator with a high efficiency regenerator.
Site constraints also usually make modifications, such as those that would permit several stages of regeneration to be achieved in a single vessel, prohibitively expensive. Part of the difficulty is that usually some form of baffling or separation is needed to achieve multistage regeneration, i.e., the fluidized bed regions must be isolated, and the flue gas from each region must be isolated. Some means of recovering catalyst from flue gas is usually essential, because even in bubbling bed regenerators with relatively low superficial vapor velocities there is a tremendous amount of catalyst entrainment into the dilute phase. Multiple cyclones in parallel, with multiple stages of cyclones, i.e., in series, are usually needed to recover catalyst from flue gas. These cyclones are heavy, and difficult to support, and when multiple stages of catalyst regeneration are involved, and great swings in temperature must be accommodated in the regenerator, the problems of cyclone support, and thermal stress, multiply.
It would be beneficial if a way could be found to convert these older, bubbling dense bed regenerators into higher efficiency units, preferably ones which could operate with relatively low pressure drop, and most preferably with staged regeneration, with isolation of each stage. This presented several complications.
It was easy to isolate the dense phase fluidized bed regions--the catalyst acted like a liquid, and a simple solid baffle would effectively isolate one fluidized region of catalyst from the other. Baffled regions, defining isolated fluidized beds sharing a common vapor region above, are common. U.S. Pat. No. 2,584,391 disclosed an apparatus with a baffled fluidized bed region which could be said to define multiple dense phase regions in a fluidized bed, but the vapor phases from each fluidized bed were mixed together and withdrawn from a single outlet. This was an improvement, it gave the option to achieve multiple stage regeneration, but added the constraint that the flue gas streams had to be compatible. If an attempt were made to operate the apparatus shown in U.S. Pat. No. 2,584,931 as a regenerator, with the inner stage in partial CO combustion mode, and the outer stage in complete CO burn mode, with an oxidizing atmosphere, the two flue gases would "afterburn" when mixed together in the dilute phase region above the dense beds. The lack of sufficient spent catalyst, to absorb the heat of combustion, would lead to extremely high temperatures in the flue gas line and in the cyclones, which could damage the unit.
Physical isolation of each combustion stage, by using, e.g., a closed first stage vessel immersed within a second stage, with cyclone separators mounted on the first stage vessel to separate catalyst from flue gas discharged from the first stage, would require considerable capital expense, an unduly large pressure drop, and cause severe technical problems due to thermal expansion. The first stage cyclones must be closely coupled to the primary stage vessel, and the cyclone outlets must be connected to the top of the vessel holding both stages. FCC regenerators are subjected to great variations in temperature, and the variations can be especially severe when multiple stages of regeneration are involved. As an example, during startup, or when a high CCR feed is first charged to the unit, the first stage of the regenerator will frequently run hotter, causing considerable thermal expansion. If the unit is all tied together, with the first stage regenerator, superimposed cyclones, and cyclone outlets passing through the top of the containment vessel, then some parts of the regenerator will be subjected to considerable mechanical stress. Expansion joints can be used, but most refiners are reluctant to use these expensive, and relatively fragile, devices within the harsh environment of an FCC regenerator.
I discovered a way to reduce the pressure drop heretofore associated with multiple bed regenerators. In a preferred embodiment, I provided a way to conduct multi-bed, and multi-stage regeneration of catalyst with relatively low pressure drop. In an especially preferred embodiment, I use cyclone separators, connective with the first stage of the regenerator, to separate catalyst from flue gas, without subjecting the regenerator to excessive thermal stress. In this embodiment, it is possible to isolate the flue gas from each stage of the regenerator so that one stage can operate in partial CO burn mode, and another can operate in complete CO burn mode.